The use of gypsum plaster in construction materials and products will always be justified by the ability of this material to quickly gain strength without heat treatment. The main restriction to an even wider application of gypsum plaster is its insufficient water resistance. The strength of moistened gypsum plaster products is significantly reduced, therefore, despite all their advantages, gypsum plaster materials are almost never used in structures exposed to significant moisture.
Recently, domestic manufacturers and researchers have developed several methods to improve the water resistance of gypsum-based materials.
One of those methods is the introduction of 15–30% (or more) of Portland cement together with active hydraulic additives into hemihydrate gypsum plaster. The resulting mixed three-component (gypsum plaster + Portland cement + hydraulic additive) binders are characterized by the rapid setting and rapid initial hardening of hemihydrate gypsum, as well as by hydraulic hardening in humid and even aqueous media (typical of cements). Studies have proven that hydraulic additives help control the interaction processes between gypsum plaster and Portland cement. Hydraulic additives reduce the concentration of calcium hydroxide in aqueous solutions, which favorably affects the formation of calcium hydrosulfoaluminate and the durability of products based on these binders over time. However, not all regions in Russia have raw materials suitable for the production of GCPB (gypsum-cement-pozzolan binders). For this reason, adding used silica gel to the binder will allow solving both the raw material and environmental issues – by recycling multi-ton production waste.
In the course of the experiments, the influence of various factors on the properties of GCPB-based samples was studied and the amount of CaO in the mixture was measured. Previous studies conducted at the Kuybyshev Moscow Institute of Civil Engineering (MISI) demonstrated the possibility of increasing the water resistance of gypsum binders by mixing them with Portland cement and active hydraulic additives. Those additives perform two main functions. First of all, they decrease the concentration of calcium hydroxide in an aqueous solution to such degree, that, due to an increase in the solubility of alumina, ettringite begins to form predominantly in the aqueous medium and not on the surface of the cement particles. As a result, ettringite strengthens the existing structure of the cement stone instead of destroying it. In this case, all ingredients of hydraulic additives that are capable of interacting with calcium oxide hydrate with the formation of poorly soluble substances have a positive impact. The second function of hydraulic additives is to bind calcium sulfates and aluminates into complex compounds that are less soluble compared to the original substances.
Thus, GCPBs are characterized by a continuous increase in strength during prolonged exposure to wet conditions, while the strength of products made of pure gypsum plaster decreases — by up to 2.5–3.0 times by the age of one month.
Mixes composed of 50–70% gypsum plaster, 20–25% cement and 15–30% hydraulic additives ensure adequate water resistance. Such composite binders are characterized by significant strength (after 1–7 days) and the ability to hydraulically harden for long periods (up to 1–2 years or more).
The use of more active tripoli or other hydraulic additives also impacts the binder properties positively. In particular, the water resistance of the binder, characterized by the ratio of the compressive strength of water-saturated samples to the strength of dried samples (DS), increases from 0.60–0.65 to 0.80 and higher.
The amount of hydraulic additive should be calculated in such a way that the concentration of calcium oxide in the mortar measured for 7 days should not exceed 0.9 g/L from the beginning of hardening and 1 g/L in the first 3 days. At a lower concentration, the properties of GCPBs are improved. In this case, binders containing low-aluminate cement will have the best performance.
The most common additive is tripoli (rotten stone). However, according to studies, its introduction into gypsum-cement compositions is not a sufficiently effective technological method to ensure optimal conditions for stone formation. Compared to tripoli, white soot (amorphous silica) is more reactive. Therefore, with a decrease in the content of hemihydrate gypsum in the system, there is no decrease in the plastic strength of the crystallization structure of the material, as is the case with the tripoli. On the contrary, the research data helped to reveal a rapid increase in the plastic strength that reaches maximum values at the 60–70% content of hemihydrate gypsum by weight of the dispersed phase. With the same content of hemihydrate gypsum in the system, an increase in white soot content of up to 10% also improves the plastic strength of the material structure. The maximum growth intensity is observed at the optimal content of hemihydrate gypsum.
With regard to tripoli, there is no such consistent pattern in the change in plastic strength. On the contrary, both with a decrease in the content of gypsum binder and with an increase in the content of tripoli, the plastic strength of the crystallization structure decreases.
The introduction of amorphous silica into gypsum-cement compositions is undoubtedly more effective than the use of active mineral additives like tripoli. To achieve the optimal structure of the stone with maximum strength, the consumption of white soot should be 10%, and for the necessary stability of this structure it should amount to 15% of the mass of Portland cement. It can be assumed that the addition of silica gel, which is a form of amorphous silica, will have the same effect on gypsum-cement-pozzolan systems as white soot.
Moreover, according to studies, silica gel which is a waste product used to eliminate gases from oil products can in fact be used as an additive. The use of silica gel can increase the water resistance of the material, regardless of the type of this substance — pure or processed. Using this technology, it is possible to obtain a water resistance coefficient of the composition of above 0.8. In this case, the material can be used not only in air medium, but also in wet conditions, as well as when exposed to water.
When the water resistance of gypsum plaster is improved with Portland cement, it is recommended to introduce electrolytes that can neutralize calcium hydroxide into gypsum-cement compositions. This is an effective technological tool that improves the conditions for the formation of artificial stone. Electrolytes make it possible to prevent the inclusion of non-hydrated particles of Portland cement in the crystallization framework (such an inclusion reduces the stability of the structure). In addition, carbonates of alkali metals intensify the processes of hydration of Portland cement, thus accelerating the hardening of gypsum-cement compositions significantly. Their quantity should correspond to the stoichiometric ratio to the free calcium oxide contained in Portland cement.
It is also recommended to perform short-term steaming of gypsum-cement building products before drying. This procedure significantly improves the quality of products: other factors being equal, an increase in strength by more than 20% is achieved. It has been established that the optimal temperature regime for the preparation and hardening of gypsum cement compositions is 35–40°C. Short-term steaming or the use of warm mixtures at such a temperature improves the conditions for hydration hardening of the cement component of the composition.
Optimizing the conditions for the formation of gypsum-cement stone makes it possible to obtain materials that, in terms of physical and mechanical properties and durability, are comparable to Portland cement-based wall materials. The use of a compound binder composed of Portland cement and gypsum plaster undoubtedly has great advantages. Due to the rapid set of strength, there is no need for heat treatment of products, which saves a large amount of heat required for this procedure. There is considerable economic benefit in replacing part of the cement with gypsum plaster due to its relatively low cost. The environmental issue of recycling waste silica gel is also being solved.